Spectral Inputs Improve Corn Crop Coefficients and Irrigation Scheduling

Abstract

ABSTRACT An algorithm was developed to shift the basal crop coefficient (K^b) curve with respect to its time axis to obtain a K^b in accordance with the real time reflectance-based crop coefficient for corn. Adjustment of the emergence and effective cover dates during vegetative growth made these dates converge on their actual occurrence. Impact of adjustments to the K^b curve on irrigation dates was simulated using the USDA-ARS irrigation scheduling model for two seasons. Simulated results for the 1983 growing season showed one or two-day differences in the occurrence of irrigation dates. Prior to effective cover, irrigations occurred earlier for the simulation using feedback (adjusted K^b). After effective cover, irrigation dates lagged for the with-feedback simulation compared to the without-feedback simulation. An extra irrigation was required prior to effective cover in 1983 for the simulation using feedback. In 1986, one less irrigation was required for the simulation using feedback to the irrigation scheduling model. This was due to K^b curve adjustments that accounted for hail damage and ensuing cool weather that slowed plant growth during early vegetative growth. The first two irrigations were delayed by two and three days, respectively, due to these events for the with-feedback simulation when compared to simulated results obtained not using feedback. Results from the simulation indicated the following: 1) revised or adjusted K^b curves derived from spectral inputs are unique to the individual growing season, 2) conventional crop coefficients contribute to underestimation as well as overestimation of crop ET because they cannot account for variable crop growth rates, and 3) adjusting the K^b curve in response to actual crop growth allows proper timing of irrigations to ensure that soil moisture conditions are ideal throughout the growing season.